The invention relates to an apparatus and method for sorting minute particles in a suspension fluid and more particularly to the fluid system for ultra high speed flow sorting. An example is an ultra-high-speed chromosome flow sorter.
There are many applications for apparatus capable of sorting minute particles entrained in a jet of a suspension fluid. Typically, the jet breaks up into droplets soon after emerging from the jet nozzle with only one particle in a droplet, and preferably only one particle in one out of 10 to 30 droplets to assure at most one particle per droplet. The criterion for sorting may be any particle characteristic that can be sensed in the jet before it breaks off into droplets. Once the characteristic of interest is sensed as a particle passes a sensing station, a decision is made to sort the droplet that will form with that particle. The sorting is accomplished by charging the droplet as it breaks off from the jet. Deflection plates down stream then cause the charged droplet to be deflected into a separate container. One important use of such a flow sorter is in the field of flow cytometry.
Flow cytometry has developed over the last two or three decades to automate counting body tissue cells in an aqueous suspension. Usually the cells have been stained to label the cellular component(s) of interest for quantitative analysis and interpretation. Briefly, the stained cells in the aqueous suspension are caused to flow under low pressure of about 10 psi through an optical sensing station. Once a particle has been sensed it can be counted and/or sorted. Typical sensing rates are 1000 cells per second, but to insure that each charged droplet contain only one cell, it is necessary to produce about 30 droplets for every cell flowing past the sensing station. Consequently, the typical droplet-production rate is 3.times.10.sup.4 per second.
It would be desirable to increase the sorting rate by a factor of 10 to 50. The rate of electro-optic or other sensing could be increased by that much since photosensitive and electronic components are available for operation at pulse rates of from 10.sup.6 to 10.sup.9 per second. The problem is with the fluidic system, and more particularly the jet velocity at 10 psi and the droplet production rate of 3.times.10.sup.4 per second are limiting factors. These have been the limiting factors in flow cytometry and sorting. A higher jet velocity requires a higher operating pressure, which compounds the problem of increasing the droplet rate, and the problem of coordinating the sort decision with the production of a charging pulse.
Although reference has been made to cells, the term has been used by way of example, and not with any intent to limit the invention to sorting only those particles that are the least structural aggregate of living matter capable of functioning as a living unit. The most likely use of this invention is in sorting mammalian chromosomes of a single type for investigations of the biochemical nature and chromosomal specificity of genetic lesions formed as a result of cellular exposure to mutogens or carcinogens. It was discovered at the Lawrence Livermore Laboratory in 1974 that chromosomes could be classified and purified to an unparalleled degree through flow sorting. The procedure is briefly as follows. Fluorescently stained chromosomes are suspended in an aqueous solution which is then surrounded by a sheath fluid in a nozzle so that the chromosomes are ejected one-by-one in a very small jet from an orifice. The chromosomes in the liquid jet pass through an intense laser beam. The resulting fluorescence from each chromosome is sensed and used to make a decision for sorting the droplets produced in response to the ultrasonic vibrations induced by a piezoelectric crystal driven at high frequencies.